Haloalkanes and Haloarenes – JEE Mains Chemistry

• by Zenith LMS
• Updated April 2025

1. Introduction

This chapter deals with compounds containing halogen atoms attached to aliphatic or aromatic carbon chains. These are known as haloalkanes (alkyl halides) and haloarenes (aryl halides).

2. Classification and Nomenclature

• Haloalkanes: Based on the number of halogen atoms: mono, di, tri, etc. Named using IUPAC rules by replacing –e of alkane with halogen prefix (e.g., chloromethane).
• Haloarenes: Aromatic rings with halogen substitution. Named using benzene as the base (e.g., chlorobenzene).

3. Nature of C–X Bond

• The carbon-halogen bond is polar due to electronegativity difference.
• Bond strength decreases from C–F to C–I.

4. Methods of Preparation

• Haloalkanes: From alcohols using HX, PCl₅, SOCl₂; from alkanes via halogenation.
• Haloarenes: Electrophilic substitution of benzene with halogens in presence of Lewis acid (e.g., FeCl₃).

5. Physical Properties

• Boiling points increase with molecular mass and are higher than corresponding alkanes due to polarity.
• Insoluble in water, soluble in organic solvents.

6. Chemical Reactions

• Nucleophilic Substitution: SN1 and SN2 mechanisms in haloalkanes.
• Reactions of Haloarenes: Less reactive toward nucleophilic substitution due to resonance and partial double bond character.
• Electrophilic Substitution: Halogens are ortho-para directing in haloarenes.

7. Mechanisms

• SN1: Two-step mechanism with carbocation intermediate; favored in tertiary haloalkanes.
• SN2: One-step mechanism with backside attack; favored in primary haloalkanes.

8. Environmental Effects

• Chlorofluorocarbons (CFCs): Cause ozone layer depletion.
• Halogenated waste: Can be persistent and toxic to ecosystems.

9. Uses and Applications

• Haloalkanes: Used as solvents, anesthetics (chloroform), and refrigerants.
• Haloarenes: Used in pesticides, pharmaceuticals, and polymers.